OCR Text |
Show 180 DH, <J. J. PORSYTH MAJOR ON MIOCENE SQUIRRELS. [Feb. 28, that the laminated hypsodont molars generally begin tubercular, and the tubercular brachydont molars, when worn, become laminated. For instance: the unworn upper aud lower molars of Castor fiber show us a somewhat tuberculate crown, in which at first sight it is not easy to recognize the well-known laminated pattern of the worn tooth of the Beaver; likewise, a much-worn molar of the brachydont Cricetus presents enamel-folds and islets, though, owing to the shallower and wider valleys, they appear less distinct than in the hypsodont molars, whose valleys are reduced to narrow but deep fissures ; so that there is only a gradational difference between tubercular and laminated teeth. The molar teeth of Sciurida? are generally represented as tubercular. But, in surveying all the known forms, even restricting ourselves to the subfamily Sciurinee, we meet with all possible intermediate stages between the decidedly hypsodont molars of Eupetaurus described by Thomasl and the utmost degree of brachydontism as shown by the molars of the Bornean Rhi-throsciurus or the Myoxine-like teeth of the group of pigmy Squirrels. First, as to Brachydontism and Hypsodontism. The species of Pteromys, in a restricted sense, are on their way to become hypsodont; they lead over on the one side to Eupetaurus, and on the other to the more or less brachydont Sciuropteri (including Pteromys tephromelas, Giinth., and P.pliosomelas, Giinth., whose molars are quite similar to each other and agree more with the Sciuropteri than with Pteromys). The African Ground-Squirrels (Xerus), as well as the Oriental Sciurus berdmorei, Bly., present a curious form of semi-hypso-dontism, inasmuch as the internal moiety of the upper and more or less the external moiety of the lower molars are more elevated vertically than the external moiety above and the internal below. Corresponding to the hypsodont part of the molar, we find ou the inner side of superior molars a stout and elongate root, on the outer side two smaller and shorter roots. A small group of Ethiopian Squirrels included in the genus Sciurus (Sc. palliatus, ccpapi, pyrropus, congicus, &c.) present a similar semi-hypsodonty, whilst the Moroccan Xerus getulus is in a lesser degree semi-hypsodont. This greater vertical elevation of the inner side of the crown in superior, and of the outer side in inferior molars, though more evident in semi-hypsodont teeth, is, however, by no means limited to them; w e meet with it, although in a lesser degree, in Sciurus vulgaris and its allies, and even in the still more brachydont Shithrosciurus. Ornithorhynchus itself, as shown by one of the figures published by Stewart2, has the inner side of the superior teeth more elongate than the outer. This cir- 1 Oldfield Thomas, "On Eupetaurus, a new form of Flying-Squirrel from Kashmir," Journ. As. Soc. Bengal, vol. lvii. ii. no. 3, 1888, pp. 250-200. 2 Ch. Stewart, " O n a specimen of the true teeth of Ornithorhynchus," Micr. Journ. vol. xxxiii. n. s. 1891, pi. viii. i. |